The invention relates to controlling transfer of an AC machine between power from a power supply and power from a variable frequency drive. More specifically, the invention is a method and apparatus for transferring an AC machine between power from a power supply and power from a drive that does not use a volts per hertz control scheme.
In many applications, it is desirable to transfer an AC machine receiving power a power supply, such as supply mains, to receive power from a variable frequency motor drive, or vice versa. The phrase "power supply", as used herein, refers to any source of AC electric power such as supply mains, a generator, an uninterruptable power supply, or the like. The phrase "AC machine", as used herein, refers to any AC rotating machine, such as AC synchronous motors, AC reluctance motors, generators, dynamos, or the like.
For example, in gas turbine static starters, a variable frequency motor drive is used in place of a diesel engine to bring a generator up to a self-sustaining speed at which the generator can subsequently be switched to supply mains. In other applications, such as variable speed fans and pumps, it is desirable to achieve variable speed with a motor drive and subsequently switch the motor to supply mains for sustained operation at a constant high speed. Also, in the event of failure of the motor drive, it is desirable to transfer the motor to the supply mains to continue operation. Similarly, it is often desirable to switch a motor from supply mains back to a motor drive for slowing the motor down or otherwise varying the speed or torque of the motor. Switching of an AC machine from a drive to a power supply is referred to as "transfer" herein and switching of an AC machine from a power supply to a drive is referred to as a "capture" herein.
Of course, AC machine speed varies with frequency of the input signal driving the motor. However, the inductive reactance of the motor drops at low frequencies, resulting in excess current in the motor if voltage is constant. Therefore, conventional V/F ("volts per frequency" or "volts per hertz") drive control, utilizes a fixed ratio between the drive voltage and frequency and the frequency is controlled under this assumption, up to a nominal operating frequency, to control the speed of a motor driven by the drive output. In V/F control drives, it is relatively simple to synchronize the frequency of the drive output and a power supply to accomplish transfer and capture. However, V/F control is open loop in nature and thus has inherent stability problems. For example, since the frequency is controlled, the machine speed is not controlled directly. Machine slip occurs depending on the load and thus motor speed is not directly proportional to frequency of the input current. Complex slip frequency algorithms have been developed to overcome this problem. However, in addition to the added complexity of slip frequency control, V/F control does not separately control the field producing and the torque producing components of the input current and thus instantaneous torque control cannot be achieved.
"Vector control" drives have been developed to overcome limitations of V/F control drives. The phrase "vector control", as used herein, refers to any type of AC machine torque control such as field orientation, natural field orientation control, and direct torque control. In vector control drives, torque in the motor is controlled directly or indirectly. Accordingly, vector control allows an AC machine to be controlled in a manner similar to a DC machine. Vector control algorithms are well known and vector control drives are used in various DC machine control applications. However, since vector control inherently does not provide a reference voltage, it is difficult to synchronize the output of a vector control drive with that of supply mains and thus it is difficult to implement line transfer or capture procedures in systems having vector control drives.